Our long-term objectives are to determine the organization of the nuclei of the lateral lemniscus and their contribution to basic auditory function.The lateral lemniscus is the main auditory fiber tract connecting the brainstem and tectum in all mammals including humans. The cell groups embedded in this tract are strategically positioned to sample from nearly the entire ascending and descending auditory data stream. The lemniscal nuclei have been implicated in such fundamental auditory functions as the ability to locate sound sources accurately and the precise analysis of the temporal fine structure of patterned sound, a critical element in human speech perception. Although loss of the lateral lemniscus results in profound auditory deficits, little is known of the specific circuitry that allows each of these cell groups to contribute to audition. This investigation will rely on a combination of intracellular and extracellular dye labeling of lemniscal neurons and associated axons in tissue slices of the auditory brainstem to address the following questions: 1) What is the dendritic morphology of cells in the nuclei of the lateral lemniscus? A systematic view of dendritic patterns is necessary to clarify the basic organization of each cell group. 2) What is the nature and extent of the local circuitry in the nuclei of the lateral lemniscus? Knowledge of the interactions within each cell group as well as interactions between nuclei is basic to understanding the nature of processing within each cell group. 3) What is the pattern of afferent axonal projections to the nuclei of the lateral lemniscus and what is the relation of specific inputs to the cellular and dendritic organization of the nuclei of the lateral lemniscus? Identification of the specific contacts formed by axons upon individual cells will provide clues to the functional organization of cell groups within the lateral lemniscus. 4) What are the patterns of efferent projections from the nuclei of the lateral lemniscus? This aim will clarify relationships among specific efferent axons, lemniscal neurons and afferent axon patterns. 5) What is the nature and extent of the convergence in the inferior colliculus between direct projections from the brainstem and indirect projections via the nuclei of the lateral lemniscus? To explore the congruence of direct and indirect axon terminals within the inferior colliculus, we will trace specific lemniscal cell axons to the inferior colliculus and compare the distribution of these axons to the distribution of ascending axons that provide both lemniscal and collicular input. 6) What is the neurochemical organization of the nuclei of the lateral lemniscus? To amplify the significance of each of the preceding aims, basic immunocytochemical techniques will be used to identify the distribution of transmitter-associated amines and peptides in the lateral lemniscus.